This invention relates to kraft pulping and in particular to the preparation of a cooking liquor which, compared to conventional kraft pulping liquor, provides an increase in pulp yield for a given degree of delignification.
In the conventional kraft pulping process, lignocellulosic materials, such as wood chips, are treated in a digester with a fresh cooking liquor (white liquor) consisting of an aqueous solution of sodium hydroxide and sodium sulfide under a certain set of conditions of temperature, pressure and time. Upon completion of the cooking schedule, the resulting pulp is washed to remove spent chemicals and dissolved wood substance. The washed pulp is screened and/or refined to remove or disperse undercooked material. The pulp is stored until drawn as required for the direct manufacture of paper products, or may be subjected to conventional bleaching treatment for the manufacture of bleached grades of paper.
The spent liquor from the pulping process is referred to as weak black liquor, which contains degraded and dissolved wood constituents and inorganic salts. This liquor is evaporated to a solids content of about 25% at which point the soaps, sodium salts of resin, and fatty acids are recovered. The strong, soap-free black liquor is further evaporated then combusted in a recovery furnace. The resulting molten mass of sodium carbonate and sodium sulfide is dissolved in water to give a "green liquor" which is treated with calcium hydroxide to convert the carbonate into hydroxide. This causticizing process regenerates a white liquor for reuse in cooking. A portion of the weak black liquor may be used to adjust the volume and concentration of the cooking liquor.
The objective of the pulping process is to chemically degrade and dissolve the lignin matrix, while preserving the carbohydrate fraction. Lignin is a high molecular weight polymer which gives wood its structural rigidity by holding the cellulose fibers together. During the course of delignification in kraft pulping, other components of the fibrous raw material are dissolved to a degree determined by the process conditions. Ideally, pulping should be conducted in a way which dissolves lignin selectively, with no dissolution or degradation of cellulose or other carbohydrate materials. There is no process which achieves this total objective.
A number of chemical additives and modifications of the pulping liquor composition have been used in the art which achieve some degree of pulp yield enhancement. In any discussion of pulp yield improvement, it should be remembered that the objective is to increase the yield of components other than lignin. It is entirely possible and common practice to increase yield by terminating the pulping reactions at an earlier stage, thus retaining more of the starting material including lignin. Yield improvement via this method involves a trade-off between yield and product quality. In practice, for any end product, there is a limiting residual lignin content above which product quality becomes unacceptable.
Among the additives and modifications in the art which have achieved higher yields at equal lignin content are hydrogen sulfide pretreatment, anthraquinone and other related quinone derivatives, and polysulfide. The use of polysulfides in the kraft pulping process is well known. A number of publications indicate that kraft pulping with sodium polysulfide provides a considerable increase in pulp yield for a given lignin content. Polysulfides catalyze the oxidation of the carbohydrates' aldehyde end-groups to form a series of aldonic acids which retard the "peeling reaction" by which carbohydrates are degraded and dissolved. Calcium-polysulfide solutions have been found to be more effective in stabilizing the carbohydrates toward alkaline degradation than the corresponding sodium-polysulfide solutions as a result of the ability of calcium ions to catalyze the formation of more alkali-stable aldonic acids.
Despite the discovery of the yield-enhancing ability of polysulfides in kraft cooking liquors, their use in kraft pulping did not gain wide acceptance in the industry since the polysulfide that must be added or generated in situ by addition of elemental sulfur, could not be recovered as such in the conventional kraft recovery system. Polysulfide is converted to its reduced form as sulfide during pulping and the feasability of regenerating polysulfide through sulfur addition is limited by the sulfur losses that would occur in the system. More recently, much interest has been generated in the use of various oxidants which allow for the formation of polysulfide through the oxidation of sodium sulfide in kraft white liquor. Among the oxidation systems developed is Mead's liquor oxidation system in which sodium sulfide is oxidized by molecular oxygen in the presence of a catalyst consisting of a granular activated carbon treated with a wet proofing agent. While this process results in the generation of polysulfide and its recovery from kraft spent liquors, operating and capital costs have been among the factors limiting its acceptance in the industry.
Degraded wood components, such as in black liquor, can also be effective as oxidation catalysts. The air-oxidation of white liquor/black liquor mixtures has been shown to generate small levels of polysulfide. The direct oxidation of cooking liquors consisting of white liquor/black liquor mixtures in the digester while in contact with the wood can lead to increased yields using moderate sulfur levels. However, the yield increases so obtained are quite small due to low levels of polysulfide formed.
Canadian Pat. No. 452910 is directed to a soda or sulphate process of pulping a batch of cereal straw wherein weak black liquor from pulping of wood is causticized with lime and then is used for cooking the straw. The interest of the patentee in the causticization of weak black liquor arises because of the economics associated with the conventional method of recovering chemicals from very dilute liquors. The pulping of bulky materials, such as straw or grasses, require much lower chemical applications than for the pulping of wood chips, so the resulting black liquors are very dilute. Thus the causticization of black liquor represents an economically attractive alternative to the conventional method for recovering chemicals. The pulping of wood chips was not included in the patentee's concept. In the examples presented, particularly example 3, no change in the sodium sulphide concentration occurred as a result of causticization, which strongly suggests that polysulfide was not generated in the system. In contrast, the present invention provides for causticization of the black liquor under conditions which involve the air oxidation of sodium sulfide to generate polysulfides. Since the presence of polysulfide is required to produce yield enhancing effect, no yield increase would be obtained from the patentee's system.
In Australian Pat. No. 225156, a cooking liquor for use in producing pulp from grasses by the soda process is obtained by regenerating and reusing the waste digestion liquor by the direct addition of lime thereto. Since it is chemically impossible to generate in situ polysulfides from the waste digestion liquor of the soda process, no yield increase could be obtained from the Australian system.